Apparatus for obviating undesirable operation of an electric circuit due to faulty switching

ABSTRACT

The disclosure relates to a gating circuit which is normally operated by means of an external switching device which supplies the necessary power to the gating circuit. A PTC element is provided in parallel with the gating circuit whereby, in the event that the switch which supplies power to the gating circuit is leaky and presents a high resistance rather than an infinite resistance in its open state, the currents provided to the gating circuit under these conditions will be shunted around the gating circuit through the PTC elements. Under operation where the switch is closed, the PTC elements will rapidly change to high resistance elements and thereby effectively remove themselves from the circuit.

United States Patent [1 1 Finnegan [451 Feb. 26, 1974 APPARATUS FOROBVIATING UNDESIRABLE OPERATION OF AN ELECTRIC CIRCUIT DUE TO FAULTYSWITCHING [75] Inventor:

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

221 Filed: Oct. 6, 1972 21 Appl. No.: 295,612

Francis Finnegan, Wrentham, Mass.

[52] U.S. Cl 307/238, 307/240, 307/310, 215/504 [51 Int. Cl. H03k 5/08,H03k 17/00 [58] Field of Search 317/234 G, 234 H; 215/504, 215/505;307/238, 239, 240, 310

[56] References Cited UNITED STATES PATENTS 2,947,844 ,8/1960 Howling219/505 3,252,052 5/1966 Nash 307/239 3,325,598 6/1967 O'Neill 307/2393,426,441 2/1969 Broski 219/505 3,467,817 9/1969 Fricker 219/5053,474,963 10/1969 Buiting et a1. 219/505 3,582,762 6/1971 Mori 317/234 H3,590,365 6/1971 Nelson 219/505 3,600,650 8/1971 Obenhaus 317/234 H3,619,560 11/1971 Buiting 219/505 Primary Examiner-John S. HeymanAssistant Examiner-Andrew J. James Attorney, Agent, or Firm-John A.l-laug; Edward .1. Connors, Jr.; James P. McAndrews 5 7] ABSTRACT Thedisclosure relates to a gating circuit which is normally operated bymeans of an external switching de vice which supplies the necessarypower to the gating circuit. A PTC element is provided in parallel withthe gating circuit whereby, in the event that the switch which suppliespower to the gating circuit is leaky and presents a high resistancerather than an infinite resistance in its open state, the currentsprovided to the.

gating circuit under these conditions will be shunted around the gatingcircuit through the PTC elements. Under operation where the switch isclosed, the PTC elements will rapidly change to high resistance elementsand thereby effectively remove themselves from the circuit.

6 Claims, 1 Drawing Figure 1 PAIENIEDFEBZBIBH I 3794.859

APPARATUS FOR OBVIATING UNDESIRABLE OPERATION OF AN ELECTRIC CIRCUIT DUETO FAULTY SWITCHING The disclosure relates to the use of a PTC elementin parallel with an electrical circuit and, more specifically, to theuse of a PTC element in parallel with an electrical circuit to preventoperation of the electrical circuit in the event that theswitch in thepower circuit supplying power thereto is faulty and provides a reducedlevel of voltage thereacross, even when in its open state.

It is well known that power is applied to electrical circuits on an onand off basis by means of switching elements such as electricalswitches, electronic switches and the like. It is often the case, due tofaulty electronic switches or dueto deterioration of poor environmentalconditions of the electrical switch, that when the switches are in theiropen state, they can merely provide a high resistance rather than anopen circuit from the source of power to the electrical circuit and theytherefore continue to provide a voltage across the electrical circuit.Often, the level of voltage supplied to the electrical circuit, evenwhen the switch is in the open condition, is sufficient to causeundesired operation of such electrical circuits. It is such undesiredoperation which it is desired to correct.

In accordacne with the present invention, the above problems of theprior art are overcome by providing an impedance in shunt with theelectrical circuit. The im pedance has the property of having a lowresistance value when a low voltage is presented thereacross. Theimpedance also has the property that when a'predetermined high voltageis placed thereacross, the resistance thereof increases dramatically.Accordingly, when the switch element is in the open state, the voltageacross both the electric circuit and-the impedance is relatively low andmost of the current is thereby shunted arognd the electric circuit andthrough the impedance element. When the switch is in the closed state,the volt age across the electric circuit and impedance element ismaterially increased, thereby substantially increasing the resistance ofthe impedance element and causing substantially all of the current fromthe power supply to pass through the electric circuit. In this way, theproblems caused by leaky switching elements as described above areovercome.

It is therefore an object of this invention to provide a protectivecircuit for an electric circuit which precludes operation of theelectric circuit due to a leaky switch at the power supply.

It is a further object of this invention to provide a PTC element inshunt with an electric circuit for shunting current around the electriccircuit below predetermined voltage conditions thereacross.

It is a yet further object of this invention to provide a circuitelement configuration which overcomes faulty operation of an electriccircuit due to faulty switches or switching elements in the power supplycircuit thereto.

The above objects and still further objects of the invention willimmediately become apparent to those skilled in the art afterconsideration of the following preferred embodiment thereof, which isprovided by way of example and not by way of limitation, wherein:

The FIGURE is a circuit diagram of a relay gating circuit with aprotective element thereacross in accordance with the present invention.

Referring now to the FIGURE, there is shown a relay gating circuit of atype known in the prior art with an impedance element in shunt therewithwhich presents a low impedance relative to the impedance of the gatingcircuit when the voltage thereacross is below some predetermined valueand presents a very high impedance relative to the impedance across thegating circuit when the voltage thereacross is above the predeterminedvoltage level. The circuit includes a relay coil 1 which is utilized tooperate external switches or other appropriate elements. Current to thecoil 1 is provided from the line 5 via a triac 3 having a gate 7thereon. The triac 3 is energized by a pulse transformer havingsecondary winding 9 and primary winding 11. Pulses are supplied from theprimary winding 11 to the secondary winding 9 of the pulse transformervia a relaxation oscillator 13 which includes the primary winding 11, a

diac 15 and a capacitor 17. The relaxation oscillator 13 A 19, whereupona pulse is provided for the relaxation 05- l cillator 13, the pulsebeing transferred from the primary winding 11 to the secondary winding 9of the pulse transformer and causing the triac 3 to operate due to theincreased voltage on the gate 7 thereof. This will allow voltage fromthe line to pass through the triac 3 and the coil 1 and allow operationof the coil in well known manner.

When the switch 19, which can be a normal electrical switch, asemiconductor switch or the like, does not present an open circuit whenin its open condition, but rather is leaky in nature (i.e., 10K ohms),current from the line 5 will pass to the line 25 through the relaxationoscillator 13, the switch 19 thereby merely acting as a high resistanceand providing a voltage which is less than the normal line voltage atthe point marked 27 in the FIGURE which is the junction of the switch 19and the resistor 21. This voltage can still be of sufficiently highvalue to cause the relaxation oscillator 13 to operate and provide apulse to winding 9, thereby providing unwanted and undesired operationof the triac 3 and the coil 1 in the same manner as described for normaloperation. Accordingly, there is provided an impedance element 29 inshunt with the entire electrical circuit composed of the components ofthe relaxation osals have a relatively constant resistance for increasein temperature until a particular threshold temperature is reachedwhereupon, at the anomaly temperature, the temperature coefficient ofthe material increases dramatically. The PTC sensors are usually formedfrom doped barium titanate, the anomalous behavior being caused by achange in the crystal structure of the doped barium titanate ceramic.The characteristic anomaly temperature and resultant temperaturecoefficient are determined by the chemical composition of the bariumtitanate. Resistance changes of the PTC devices in a steep slope regionafter the anomaly temperature has been reached fall in the range of 25to percent per degree centigrade. The anomaly temperature herein isreached due to the R heating therein generated by the increase in thevoltage thereacross.

When the switch 19 is open, the voltage across the impedance element 29as well as the electric circuit will be relatively low, even in theevent of leakage. In this event the resistance of the impedance element29 will be low compared to that of the electric circuit composed of theoscillator 13 and resistors 21 and 23. The impedance 29 will thereforeact as a shunt and cause current to flow substantially therethroughrather than through the electric curcuit, thereby preventing any falseoperation of the electric circuit. When the switch 19 is closed, theentire line voltage is placed across both the electric circuit and theimpedance element 29. At this voltage, the PTC element will heat up andsubstantially increase its resistance, whereby a substantial part of thecurrent from the line will pass through the electric circuit and causenormal operation thereof. It can therefore be seen that the particulartype of impedance element utilized herein will substantially eliminatefaulty operation of the electric circuit due to the inability tocompletely switch off power from a power supply.

The capacitor 31 and resistor 33 which are in shunt with the triac 3 areplaced in the circuit to prevent operation of the triac due to spurioustransients in the line. The resistor 35 performs the same function.

Although the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications thereofwill immediately become apparent to those skilled in the art. It istherefore the intention that the appended claims be interpreted asbroadly as possible in view of the prior art to include all suchvariations and modifications.

What is claimed is;

1. A circuit operation connection device which comprises, incombination: a. an electric circuit,

b. switch means coupled to said electric circuit, 0. a source of powercoupled across said electric circuit and said switch means, and d.impedance means connected in shunt with said electric circuit, having alow resistance relative to said electric circuit when the voltage acrosssaid electric circuit is below a predetermined value and having a highresistance relative to said electric circuit when the voltage acrosssaid electric circuit is above said predetermined value. 2. A circuit asset forth in claim 1 wherein said impedance means is a PTC device.

3. A circuit as set forth in claim 2 wherein said PTC device is a dopedbarium titanate device wherein the Curie point of said PTC device isreached when a voltage of at least said predetermined voltage is placedthereacross.

4. A circuit as set forth in claim 1 wherein said switch means has openand closed states, said switch, when in its open state providing avoltage less than said predetermined voltage, across said electriccircuit.

1. A circuit operation connection device which comprises, incombination: a. an electric circuit, b. switch means coupled to saidelectric circuit, c. a source of power coupled across said electriccircuit and said switch means, and d. impedance means connected in shuntwith said electric circuit, having a low resistance relative to saidelectric circuit when the voltage across said electric circuit is belowa predetermined value and having a high resistance relative to saidelectric circuit when the voltage across said electric circuit is abovesaid predetermined value.
 2. A circuit as set forth in claim 1 whereinsaid impedance means is a PTC device.
 3. A circuit as set forth in claim2 wherein said PTC device is a doped barium titanate device wherein theCurie point of said PTC device is reached when a voltage of at leastsaid predetermined voltage is placed thereacross.
 4. A circuit as setforth in claim 1 wherein said switch means has open and closed states,said switch, when in its open state providing a voltage less than saidpredetermined voltage, across said electric circuit.
 5. A circuit as setforth in claim 2 wherein said switch means has open and closed states,said switch, when in its open state providing a voltage less than saidpredetermined voltage, across said electric circuit.
 6. A circuit as setforth in claim 3 wherein said switch means has open and closed states,said switch, when in its open state providing a voltage less than saidpredetermined voltage, across said electric circuit.